What Is TypeScript?

JavaScript never checks whether the values flowing through your code are the kind you expect until the moment that code actually runs. TypeScript is a superset of JavaScript, created by Microsoft, that layers a type system on top of it — so a whole category of mistakes gets caught the moment you save the file, before you ever hit run.

function greet(name: string) {
  return "Hello, " + name.toUpperCase();
}

greet(42); // Error: Argument of type 'number' is not assignable to parameter of type 'string'.

That's the core idea: TypeScript checks types at compile time — while you're writing the code — instead of at runtime, while it's running on someone's machine. The earlier a bug is caught, the cheaper and less embarrassing it is to fix.

Types also make your editor smarter. Once TypeScript knows a variable's exact shape, it can offer accurate autocomplete, flag typos in property names, and show you exactly what a function expects — all without running a single line of code.

const user = { name: "Ravi", age: 29 };

user.ag; // Editor flags this immediately: Property 'ag' does not exist on type '{ name: string; age: number; }'. Did you mean 'age'?

Here's the part that surprises a lot of beginners: browsers and Node.js can't run .ts files directly. TypeScript is compiled — translated — down into plain JavaScript before it ever runs. The type annotations only exist to help you while you write; by the time the code actually runs, they've been stripped away entirely.

TypeScript is a ____ of JavaScript that adds a type system on top of it.

Why does TypeScript catch more bugs than plain JavaScript before the code ever runs?

Browsers can execute .ts files directly, without any compilation step.